1. Field of the Invention
The present invention relates to a catalyst system and a process for combined hydrotreating and hydrocracking operations in a single reactor bed by contacting a hydrocarbonaceous feedstock with hydrogen under hydrocracking conditions in the presence of an appropriate dual function catalyst system. In particular, the catalyst system and process of this invention relate to a combined denitrification and/or desulfurization hydrotreating process and a hydrocracking process wherein the catalyst system exhibits surprising stability and high selectivity for liquid products boiling in the transportation fuels range. The catalyst system can be tailored to provide previously unavailable flexibility with regard to the selection of the hydrocracking catalyst.
The dual function catalyst system of the present invention comprises two randomly intermixed particulate catalysts having distinctly different catalytic functions. The first catalyst is a conventional hydrodenitrification and/or hydrodesulfurization catalyst having substantially no cracking activity. The second catalyst is a conventional zeolitic hydrocracking catalyst. Both catalysts are selected so that they are substantially the same size, that is, the effective diameter for each catalyst particle is substantially the same.
The novel catalyst systems of the present invention have been found to provide surprisingly good selectivity for liquid products and stability against catalyst fouling when used in combined hydrotreating and hydrocracking applications, and can therefore be used to provide a stable catalyst system which offers even heat distribution and reactor control in such applications.
2. Objects of the Invention
Of the many hydroconversion processes known to the petroleum refining industry, catalytic hydrotreating and catalytic hydrocracking are perhaps the two most widely applied and important. In conventional refining practice, hydrotreating is carried out using a catalyst(s) having as the principle function the removal of nitrogen and/or sulfur, that is catalytic hydrodenitrification and hydrodesulfurization. The product of hydrotreating is then fed to a hydrocracking process unit which uses catalysts having as the principle function hydroconversion to produce liquid products boiling in the transportation fuels range.
Hydrotreating the feedstock to a hydrocracking process unit is particularly important as nitrogen and sulfur are known to contaminate conventional hydrocracking process catalysts. Thus, hydrotreating is used to lower the nitrogen and sulfur content of the hydrocarbonaceous feedstock stream to an acceptable level before subjecting the hydrocarbons to the complete hydrocracking process. In general, it is desirable to lower the nitrogen content of the hydrocarbon feedstock stream to less than 50 parts per million by weight (ppm), preferably less than about 10 ppm and in many cases for increased catalyst life to a level of less than 2 ppm or even as low as about 0.1 ppm. Similarly, it is generally desirable to lower the sulfur content of the hydrocarbon feedstock stream to less than about 0.5% by weight percent, preferably less than about 0.1%, and in many cases as low as about 1 ppm.
However, hydrotreating catalysts have various disadvantages. Perhaps the most noted disadvantage is the tendency to foul with coke or other contaminants at an excessive rate. This results in shorter catalyst life than is desirable. As the catalyst fouls or deactivates, the denitrification process temperature must be increased to maintain activity. When the maximum temperature allowed by process and equipment limitations is reached, the catalyst must be replaced or regenerated.
A variety of measures have been suggested to overcome the problems of catalyst deactivation in hydrotreating systems. For example, U.S. Pat. No. 4,990,243 issued Feb. 5, 1991 to Winslow describes a layered catalyst system for hydrodenitrification. The idea behind layered systems is to provide a catalyst system which permits the operator to control the process conditions such as temperature to allow more uniform operations while removing contaminants such as nitrogen. In particular, the layered systems utilize discrete catalyst layers with differing catalysts having differing activity for denitrification and cracking. The first layer is a more active denitrification catalyst which does not induce cracking reactions. The second layer is more acidic and has higher cracking activity which results in effective conversion of the refractory nitrogen compounds not converted in the first layer.
U.S. Pat. No. 4,534,852 issued on Aug. 13, 1985 to Washecheck et al. describes a single stage hydrotreating process for converting pitch to conversion process feedstock. According to this process the pitch containing feedstock is contacted with hydrogen and passed downwardly through a hydrotreating zone over a stacked-bed catalyst. The upper bed contains a high activity hydrotreating catalyst, and a separate lower bed contains a high activity desulfurization catalyst. The reaction product is a suitable hydrocracking feedstock.
U.S. Pat. No. 3,923,638 issued on Dec. 2, 1975 to Bertolacini et al. describes a two-catalyst hydrocracking process. In this process a nitrogen containing feedstock is denitrified in a pretreatment zone using a hydrodenitrification catalyst. The denitrified effluent is passed to a hydrocracking zone. The process can be carried out in a single stage.
As noted previously the product from hydrotreating can be fed to a hydrocracking process unit. Modern hydrocracking catalysts are generally based on zeolitic materials which may have been adapted by techniques like ammonia ion exchange and various forms of calcination in order to improve the performance of the hydrocracking catalysts based on such zeolites. In nearly all cases, hydrocracking catalysts are formulated to provide varying degrees of cracking activity depending upon the desired product slate. Thus, hydrocracking catalysts which have high activity, and therefore promote the exothermic cracking reactions, may not be suitable for all applications.
Accordingly, the general approach of catalyst manufacturers has been to offer a family of catalysts tailored in activity for various applications. In other words, operating flexibility is achieved by selecting from a variety of available catalysts the one catalyst which is most suitable for the specific application at hand. However, this solution has created another difficulty. Refiners have found that on occasion the product slate changes which they wish to make are not possible if the choice of available hydrocracking catalysts in inventory does not include the particular catalyst with the activity required to produce the new product slate.
Thus, it would be desirable to provide a stable hydrotreating catalyst system with high denitrification and/or desulfurization activity which could be used to produce a low nitrogen low sulfur feedstock to a hydrocracking process. It would also be desirable to provide a flexible hydrocracking catalyst system which had high selectivity for liquid products.
It would be even more desirable to provide a stable catalyst system which could be used to simultaneously carry out combined hydrotreating and hydrocracking to selectively produce liquid products in the transportation fuels boiling range.
Several attempts have been made to provide dual function combined hydrotreating and hydrocracking processes and catalyst systems.
U.S. Pat. No. 4,797,196 issued on Jan. 10, 1989 to Kukes et al. describes a hydrocracking process having intermixed catalysts. In this process, each of the intermixed catalysts has hydrodenitrification and/or hydrodesulfurization activity as well as cracking activity, that is they both have zeolitic components and function to crack the feedstock. Thus, although one of the catalysts is predominantly a hydrotreating catalyst, each catalytic particle is dual functional.
U.S. Pat. No. 4,210,521 issued on Jul. 1, 1980 to Gorring et al. also describes a dual bed catalytic upgrading process for refractory hydrocarbon stocks. In this process, the refractory feedstock is first catalytically hydrotreated and the hydrotreated product is subsequently cascaded through a hydrocracking zone. The initial hydrotreating step serves to convert sulfur and nitrogen derivatives of hydrocarbons to hydrogen sulfide and ammonia while depositing metal contaminants.
U.S. Pat. No. 4,363,719 issued on Dec. 14, 1982 to Bousquet et al. describes a process to improve the stability of a catalyst to be used for lowering the cloud or turbidity point and the filterability limit temperature of gas-oils. The catalyst is a composite of a non-acidic hydrodesulfurization catalyst and a non-zeolitic silica-alumina based hydroconversion catalyst.
It is the principal object of the present invention to provide a stable catalyst system for combined hydrotreating and hydrocracking process operations with high selectivity for liquid products in the transportation fuels boiling range. This and other objectives are accomplished by the catalyst system and process summarized below.